In an attempt to provide metal sheet on which aesthetic appearance can be made, various techniques have been developed, and the following methods have been in the actual use. In one method, a resin coating is applied to the surface of a plated or non-plated metal base material. After that, beautiful colors, designs or patterns may be printed thereon. As a result, a beautifully coated metal sheet is obtained. In another method, a metal base material is subjected to hot rolling, and a black oxide layer naturally formed during the hot rolling is utilized to make designs thereon. There is also a method for providing a steel sheet or a titanium alloy sheet formed with decorative coatings in various colors. In this method, an oxide layer of navy blue or other colors is artificially formed onto the surface of the metal base material. The metal sheets obtained in these conventional methods have beautiful and aesthetic appearance formed; however, a coolness and touch feel inheriting from metal sheets remain.
On the other hand, film-laminated metal sheets are also known. The film-laminated metal sheets are formed by laminating a film of polyvinyl chloride, polyolefin, or an acrylic resin onto a metal base material. The laminated films are effective in giving high corrosion resistance and scratching resistance to the metal sheets; however, few of them have aesthetic appearance property. In addition, these laminated films are thermoplastic in applications where the processability is important, and therefore, they have low heat resistance. As laminated films, vinyl polyvinyl chloride films have been widely used; however, they generate harmful gas when burned, causing an air-pollution problem.
In order to solve the above-described problems of the prior arts, studies have been proceeded to develop a new type of laminated metal sheets. For example, Japanese Unexamined Patent Publication No. 7-256822 discloses a paper-laminated metal sheet including a metal base material and paper laminated on the metal base material via an adhesive agent. The features of this invention seem to reside in that, when the paper-laminated metal sheet is subjected to a bending process, the portion to be bent is wet -with water and then is bent, thereby preventing the paper from being torn. However, the step of wetting the portion to be bent of the paper-laminated metal sheets one by one makes the production process complicated, and lowers the productivity. In addition, whereas the wet paper-laminated metal sheet can easily bent, the strength of its paper is lowered. As a result, a problem arises in that the paper is damaged or torn during the processing. There is also a problem that, in the state of being wet with water, the paper loses its adhesiveness to the metal base material. Depending on the processings, the paper may peel off from the metal base material.
The present invention has been achieved in view of the above-described situation, and an objective thereof is to provide a paper-laminated metal sheet which can be processed without wetting with water, while preventing the paper from being torn with, and which keeps the aesthetic appearance derived from the paper for a long period of time.
Another objective of the present invention is to provide a paper-laminated metal sheet with high strength and processability derived from the metal base material, as well as unbelievably high aesthetic appearance for a metal plate derived from paper, whereby the paper-laminated metal sheet is valuable as a novel material having various applications.